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Tender and sore nipples are one of the symptoms suggestive of pregnancy. Look out for development of any other new symptoms suggestive of early pregnancy. If it is not constant but periodic, then it could be associated with breast and nipple sensitivity associated during ovulation due to hormonal changes.
Consider visiting your doctor/gynecologist to confirm the cause for your symptoms. If it is not due to pregnancy, and is increasing in intensity, then you might be put on medications for relief.
Take care.